The lecture introduces calorimetric investigations of phase transitions in which electrons are directly involved and discusses the mechanisms governing those phase transitions: (i) spin crossover phenomena occurring between high- and low-spin states in transition-metal complexes, (ii) intramolecular electron transfers in mixed-valence complexes, (iii) neutral-to-ionic transitions due to charge transfer mechanism, and (iv) thermochromic phenomena due to a change in coordination geometry. Since a change in the electronic state is strongly coupled with a change in the lattice. it turns out that the transition mechanisms always include interplays with various molecular motions in the lattice. In all these systems, a large change in the electronic energy is compensated by the entropy term due to these molecular motions.